1. Field of the Invention
An object of the present information is a method for the distribution of binary information at high bit rates. It is aimed at using existing infrastructures to transmit information. The invention is more particularly useful in a small environment even if its use can be envisaged in a very big geographical area
2. Description of the Related Art
In the field of information transmission, the article by John M. Cioffi et al, xe2x80x9cVery High Speed Digital Subscriber Linesxe2x80x9d the IEEE Communications Magazine, April 1999, pp 72 to 79, lists all the techniques that can be used to transmit information at a high bit rate on a subscriber line consisting typically of a pair of copper wires, especially twisted copper wires. It is known that successive statements have been issued in this field, laying down the limits of transmissible bit rates and extending these limits each time. In practice, in these techniques, we distinguish between HDSL (High Speed Digital Subscriber Line) techniques that can be used with bit rates of the order of 100 Kbit per second over hundreds of kilometres, ADSL (Asymmetrical Digital Subscriber Line) techniques which are asymmetrical subscriber lines permitting bit rates of 1 to 2 Mbit per second over several kilometres, and VDSL (Very High Speed Digital Subscriber Line) techniques that allow bit rates of about 10 Mbit per second over very short distances, for example distances of 300 m.
Furthermore, especially in the field of home automation, there are known infrastructural problems related to the installation of high bit-rate connections in houses and office buildings. Indeed, the arrival of a coaxial cable head, a twisted pair, an optic fibre or a satellite port for television distribution opens the door to uses of the Internet through the access offered by the cable operator who has installed the cable. The drawback of this technique is that the coaxial cable thus brought in provides only one head, i.e. only one place of use. In home use, it may be useful to have a pending connector available at several points in a house for the connection, on demand, of a piece of equipment or making it possible, with this connection, to obtain a high bit rate accessible with a coaxial cable of this kind. A coaxial cable of this kind may furthermore be used on a multipoint basis by providing each end that is left unconnected with a circuit having a characteristic impedance. These circuits generally take the form of a plug that can be connected to a BNC type connector. This type of distribution is used especially in the field of office information technology systems for small local-area networks.
The drawback of this coaxial cable technique however is that the distribution of the access points through a dwelling or a building implies infrastructural works, cable passages and the making of connection outlets.
There are also possibilities of using a telephone line that comes into a subscriber""s home. In this case two problems persist. Firstly, the distribution of the access points is not as general as users might wish even if, in modern dwellings, the presence of telephone sockets in each room of the dwelling is becoming a norm. However, this method of transmission suffers from the existing passband limits in the subscriber line concentrators used to connect several dwellings in an area of dwellings to a telephone exchange. In practice, the circuits of the concentrators have passbands that are below 100 KHz, thus making it possible to attain the high bit rates envisaged above, namely bits rates of the order of 10 Mbits per second.
To resolve this problem, the invention makes use chiefly of a technique of carrier currents. To set up a carrier current channel, it is necessary first of all that a sender unit or a receiver of signals to be transmitted should be coupled to an existing conductive line, generally the electrical power supply line of a building, or possibly on of the many telephone distribution lines subdivided into a tree structure in this building. A coupling of this type is known. It has the effect of isolating the sender unit and the receiver from the power signals distributed on the line which, in the present case, are of the order of 50 Hz for the electrical power supply or from the telephone signals whose frequency is below 30 KHz. At the same time, it has the effect of giving rise to an electrical decoupling in order to prevent the high voltage present on the line, an electrical supply voltage of about 220 Volts or 110 Volts, or an 80-volt pulse voltage in the case of a ringing sound conveyed by a telephone line, from disturbing the working of this sender unit and its receiver. Furthermore, a coupler of this kind couples the line in use, which may be the electrical power supply line or the existing telephone line, of the information to be transmitted or received.
In this case, the passband of this transmission is offset. In a preferred example of the invention, this passband will range from 150 KHz to a little more than 5 MHz. Indeed, beyond 5 or 6 MHz, the carrier current channel does not propagate the signals properly unless very complex solutions are contrived. In the invention, binary information elements at high bit rate are then received at a point of access to a global network, especially by a coaxial cable, an optical fibre, a twisted pair or the like, in a specialised receiver, for example a decoder provided by a cable operator. This binary information at high bit rate may be, firstly, television signals in the known passbands and, secondly, data signals resulting for example from an Internet connection on the part of the user and a transmission of the results of a request made by this user. The decoder or master device of the invention decodes the received signals and sends them to a user terminal, typically a microcomputer in the context of an Internet use, by means of a carrier current link. This network master device thus distributes the access to the total network, throughout the building, in carrier currents.
In this respect, an object of the invention is a method of distribution of binary information elements at high bit rate coming from a remote sender unit wherein:
a stream of information elements is received in a specialised receiver linked to the remote sender unit,
this stream of information elements is transmitted from the specialised receiver to a user terminal located in the vicinity, characterised in that:
the stream of information elements, after decoding in the specialised receiver, is retransmitted between the specialised receiver and the user terminal on a network, for example an electrical power supply network.
In another field, namely that of DMT or Discrete MultiTone mode modulation, there are known ways of transmitting information elements between a sender unit and a receiver by separating a wide passband, herein typically ranging from 150 KHz to more than 5 MHz in the example, into a large number of contiguous elementary frequency bands. In the example which will be taken hereinafter in the invention, a number of frequencies equal to 128 is chosen. The width for each of these frequencies is 39.0625 KHz giving a total bandwidth of 5 MHz. In this context, a specific problem appears because of a tree-structure network such as an electrical power supply mains network in a house. This specific problem is that of the numerous reflections to which the terminations of this network subject the signals that propagate therein. The channel thus constituted between two given points of this network is not very comfortable to use because it is the site of these reflections, also because it has a changing nature and finally because it is highly noise-ridden.
It is furthermore changing in nature because it is enough for the user to connect an electrical instrument to a current outlet or, quite simply, light up a lamp for the circuit of the distributed reflections to be overturned and cause other disturbances or different disturbances. Finally, a carrier current channel of this kind is noise-ridden because certain machines such as vacuum cleaners, washing machines or heating installations, when starting, have cosine xcfx86 values different from 1 and are therefore the site of noises whose duration is typically in the range of one millisecond to ten milliseconds.
It is also possible to distinguish narrow-band interference noises resulting from public or institutional sender units sending in a band of about 200 KHz for which the distributed network is a pick-up antenna. Furthermore, at a frequency of 150 KHz, the intervention of noises caused by radio amateurs is noted. The above-quoted article discusses the intervention of these noises in certain frequency bands of the multifrequency channel. Whereas, theoretically, the interference should be limited to a single subband, in practice it has been shown that numerous related bands are also disturbed.
These channel defects are echoed in a pulse response of the carrier current channel typically leading to a build-up time of about 15 microseconds. To resolve the problem inherent in this non-zero response time, there are known ways of transmitting binary information elements of the blocks and separating the blocks by a guard time equal to the duration of the pulse response of the channel. The guard time thus constituted then acts on the whole as factor of reduction of the useful bit rate. As shall be seen hereinafter, it will become necessary to provide for information blocks whose transmission time is in the range of 25.6 microseconds. In this case, the presence of a 15-microsecond guard time is incompatible with use: it makes the transmission capacity of the channel drop by almost 50%. The problem entailed by this lengthy pulse response duration is then resolved by placing time equalizers (TEQ) in the circuits connected to such channels.
The purpose of time equalizers such as these, briefly, is to prompt a sending delay for certain spectral components that propagate more quickly (for example, high spectral components) as compared with ahead-of-time sending for other spectral components (for example low spectral components). The result thereof is that, perceived from the terminal, the signals all seem to arrive at the same time, without undergoing the effects of intersymbol interferences from this slow pulse response of the channel. In practice, it is possible to easily lower the pulse response of the signal from fifteen microseconds to five microseconds, but not to zero microseconds which would be the ideal solution.
In the invention, this problem is also resolved by placing time equaliser circuits on the transmission path. However, owing to the complexity of these temporal equaliser circuits, their cost is high. To then substantially reduce the cost of the installation, it is planned in the invention to preferably use only one time equaliser circuit per installation. This equaliser circuit will be installed in a network master circuit, typically the decoder envisaged further above. In its downward relationship with user terminals, the equaliser circuit that makes an a priori time equalisation: the high-frequency components are delayed before they are sent. In the other, upward direction, when the user terminal sends information to the central circuit, the decoder, the equaliser circuit delays the high-frequency components before processing them. Thus, the communication between the decoder and the user terminal may take place as if the response time of the channel were reduced. Typically, it will be reduced to less than 5,6 microseconds instead of the known duration of 15 microseconds.
In this case, if there are several user terminals and if these terminals are to be capable of exchanging information with one another, the high bit rate of transmission between them will be obtained by preliminary transmission to the master circuit and by the re-sending, by the master circuit to another user terminal, of information sent by a first user terminal. However, even by acting in this way, the computation time of the equaliser circuit is still high. Indeed, this computation must be done at each major change in the pulse response of the channel, which is particularly frequent in carrier currents. In the invention, this computation time is reduced by simplifying this equaliser circuit, in bringing it into play only for a limited number of spectral components, namely those for which the signal-to-noise ratio is the best. It will be shown that this action brings the desired result at lower cost.
An object of the invention therefore is a method for the distribution, at high bit rate, of information coming from a sender unit, in which:
a channel is used to send a stream of information between a sender unit and a receiver, especially by a carrier current channel formed by an electrical power supply network,
the information elements of the stream of information are modulated with a DMT or discrete multitone mode of modulation that is preferably synchronised and preferably on 2N carriers, and they are demodulated correspondingly in the receiver,
the channel is time-equalised, characterised in that
the channel is time-equalised for some of the frequencies for which the signal-to-noise ratio is more favourable that it is for other frequencies.
From this viewpoint, the sender unit may even be located in a satellite. In this context of DMT mode modulations, it is also known that one of the frequency bands is designed to convey a pilot signal, typically a signal at a fixed frequency, loaded with no information but useful especially to perform a synchronisation. In electrical type distributed networks, it is also necessary to convey signalling information, or even addressing information, at low bit rates. Signalling signals such as these are, for example, remote control signals used in home automation installations for the putting into operation, from a central panel, of a washing machine at a pre-planned time, or an oven or else again for turning off a central heating system in certain rooms only when the dwellers are absent. Or else, a panel of this kind will be used to manage the remote monitoring and operation of the intrusion detectors in the building. Despite the diversity of this type of equipment, the quantity of information to be transmitted is small. It does not require the bit rates referred to here above and the transmission is generally by specialised links (which themselves imply a modification of infrastructure through the passage of specialised wires).
In the invention, this problem is resolved without the need to draw any specialised lines by taking advantage, in the context of a modulation in DMT mode, of the presence of a pilot signal. In practice, the invention uses a frequency band adjacent to that of the pilot signal and a signal conveyed by this adjacent frequency band is compared with the signal conveyed by the pilot frequency band. In the adjacent band, the modulation is preferably a PSK or Phase Shift Keying type of modulation. It will then be shown that it is enough, during one and the same period, to measure the signals delivered in these two frequency bands, multiply one of them by the other, transmit the results of this multiplication to an accumulator and test the result of this accumulator at selected points in time to deduce the phase therefrom and, therefore, the information conveyed by the adjacent frequency band. A low-cost means is then obtained for distributing information elements at low bit rate.